(a) Technical Field
The present invention relates to a fuel cell electrode and a method for manufacturing a membrane-electrode assembly (MEA) using the same. More particularly, it relates to a fuel cell electrode, which has excellent physical and chemical durability, and a method for manufacturing a membrane-electrode assembly (MEA) using the same.
(b) Background Art
A fuel cell stack, which generates electricity in a fuel cell system, has a structure in which several tens to several hundreds of unit cells, each comprising an MEA and a separator, are stacked together.
The MEA comprises a polymer electrolyte membrane, as well as a negative electrode and a positive electrode, which are disposed on either side of the polymer electrolyte membrane. The negative electrode (also known as the “hydrogen electrode”, “fuel electrode”, “anode”, or “oxidizing electrode) and the positive electrode (also known as the “air electrode”, “oxygen electrode”, “cathode”, or “reducing electrode”) are configured so that a catalyst layer including platinum catalyst nanoparticles is formed on an electrode backing layer, which may include, for example, carbon paper or carbon cloth.
Conventional methods for manufacturing membrane-electrode assemblies will be described below. As shown in FIG. 1, a catalyst slurry is coated, sprayed or painted on a gas diffusion layer to form an electrode, and the electrode is bonded to a polymer electrode membrane by thermal compression. Alternatively, as shown in FIG. 2, a catalyst slurry is coated, sprayed or painted directly on a polymer membrane and the resulting polymer membrane is bonded to a gas diffusion layer. In another alternative, as shown in FIG. 3, a catalyst slurry is coated, sprayed or painted on a release paper and transferred to a polymer membrane to form an electrode, and the electrode is bonded to a gas diffusion layer.
The aforementioned conventional art methods suffer from numerous disadvantages. For example, when the catalyst slurry is applied to the gas diffusion layer, it becomes difficult to manufacture the MEA consequently, this method is not commercially viable. A further disadvantage of the method of directly forming the catalyst layer on the polymer membrane is that it becomes difficult to manufacture an electrode with a large surface area due to deformation of the polymer membrane. Another disadvantage of the method of forming the catalyst layer on the release paper and transferring the catalyst layer to the polymer membrane is that the catalyst layer may be cracked depending upon the thickness of the catalyst layer, the content of a binder, and the type of the catalyst; consequently, the catalyst layer may be lost during transfer to the polymer membrane. Moreover, after the catalyst layer is transferred to the polymer membrane, cracks may be formed in the catalyst layer such that the polymer membrane is directly exposed to the gas supply channel of the separator through the cracks, thereby deteriorating the performance and durability of the fuel cell.
Another factor that decreases the durability of the manufactured MEA is that the polymer electrolyte membrane is broken down due to chemical instability, which occurs during both operation and idle states of the fuel cell. Moreover, the breakdown of the polymer electrolyte membrane is caused directly by hydroxyl radicals (OH radicals), which are generated by hydrogen peroxide, which is produced when oxygen or hydrogen diffuse through the polymer membrane, and also during the reaction at the oxygen electrode. The hydroxyl radicals break down the functional group (—SO3H) at the end of the polymer electrolyte (binder), which serves to decrease the conductivity of hydrogen ions, thereby deteriorating the performance of the fuel cell.
Another factor that decreases the durability of the manufactured MEA is that the voltage and current of the vehicle fuel cell are significantly altered by the operating conditions of the vehicle. For example, significant changes in the voltage of the fuel cell frequently occur during vehicle operations such as starting, stopping, accelerating, decelerating, etc. As a result, the catalyst deteriorates more rapidly, thereby reducing the durability of the fuel cell. In particular, such voltage changes have a greater effect on the cathode than in the anode; consequently, the growth, dissolution, and agglomeration of catalyst particles occurs to a more significant degree in the cathode, thereby reducing the performance of the fuel cell.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention.